In the field of information and communication, the communication traffic for exchanging a large amount of data at a high speed by using light has been improved rapidly in recent years. Particularly, the use of broadband access lines has been accelerated in accordance with the explosive growth of the Internet, and FTTH (Fiber To The Home) services have been significantly established in the market. Among optical transmission systems of FTTH, the PON (Passive Optical Network) system, which allows plural subscribers to share one optical fiber, is now enjoying an increasing demand. The PON system receives data transmitted from a central office through one optical fiber, branches the received data to 16 to 32 optical fibers by using a splitter, and distributes the data to each subscriber's home or office, thereby greatly reducing the cost of laying optical fibers.
An ONU (Optical Network Unit) is installed at a subscriber's home or office as a terminal device. The ONU subjects a downlink signal (having a wavelength of 1.5 μm), which is transmitted from the central office to the subscriber's home or office, and an uplink signal (having a wavelength of 1.3 μm), which is transmitted from the subscriber's home or office to the central office, to wavelength division multiplexing (WDM), and transfers the uplink and downlink signals with the same optical fiber. A dual-wavelength bi-directional optical module is mounted in the ONU. This optical module basically includes a laser diode (LD) for transmitting an uplink signal, a photodetector (PD) for receiving a downlink signal, and a WDM filter for separating uplink and downlink signals.
A conventional module system is shown in FIG. 9. This figure shows a basic configuration of a single-conductor bi-directional (BIDI) module. This module includes a laser diode 175, a photodetector 172, and a wavelength-selective filter 177. These optical parts are spatially arranged in a package 178. The use of this conventional module system makes it possible to manufacture the optical parts independently. Therefore, it is easy to achieve an adequate manufacturing yield. Further, an optical connection can be established by means of so-called active alignment, which is effected to achieve optical axis alignment with an optical fiber 170 while operating the optical devices 175 and 172 mounted in CAN packages 173, 176 in which lenses 171, 174 are respectively integrated. This provides an advantage of achieving stable optical coupling efficiency. At the same time, however, this also increases the number of parts and the number of processing man-hours, thereby making it difficult to achieve downsizing and cost reduction.
FIG. 10 shows a basic configuration of a second system of a single-conductor bi-directional module disclosed in Non-patent document 1 (IEICE Technical Report, Vol. 107, No. 7, R2007-2, pp. 7-10). The example shown in this figure indicates that downsizing is achieved by placing a laser diode 182, a photodetector 186, and a wavelength-selective filter 183 in a CAN package 187.
However, as is the case with the first example described earlier, it is necessary to arrange the laser diode 182, photodetector 186, and wavelength-selective filter 183 in a three-dimensional pattern. As downsizing is achieved, it is necessary to mount the above parts with increased accuracy. Further, the process of axis alignment becomes complicated. Moreover, when, for instance, a triple-wavelength bi-directional optical module is to be manufactured in consideration of extensibility, it is necessary to increase the number of optical parts and the mounting area at least twofold. This makes downsizing and cost reduction more difficult.    Non-patent document 1: IEICE Technical Report, Vol. 107, No. 7, R2007-2, pp. 7-10